If objects made of metal materials are introduced into the area of such a field, eddy currents are induced in these which lead to heating dependent on the material, time of exposure and level of field strength. In the presence of such conditions, such an object can reach a temperature which can lead to damage, e.g. to melting of plastic surfaces, or to hazards to persons. The latter occur in particular when the secondary side has been removed and heated metal objects are freely accessible and can be touched by persons.
Due to the characteristics of existing applications for inductive energy transmission systems, such a hazard arising from metal foreign bodies was assessed as not being relevant or, for example in the case of industrial trucks (AGV) attempts were made to remove such objects from critical field areas by fitting brushes in front of the secondary consumers. In the case of vehicles with a driver, training can be conducted to draw attention to the need to look out for the presence of such objects while in operation and to ensure that these are removed before the inductive transmission is set in operation, or in the event of any doubt to ensure that the inductive transmission is not set in operation. The existing safety measures appear unsuitable or at least inadequate for largely automatic operation or with higher safety requirements which must be assumed in particular when such systems are used in areas accessible to the public.
From US 2007/0145830 A1 a system for the wireless transmission of electrical power to electronic devices is known, which comprises a plurality of primary coils, thereby eliminating the need for precise alignment of a primary and a secondary coil relative to each other. In this document, the problem of the presence of metallic objects is mentioned, but a metal detector is dismissed as being unusable. Rather, by suitable circuit design it is ensured that the system only enters its resonant state when a resonant circuit comprising a secondary coil and a tuning capacitor connected in parallel approaches the primary side, thereby causing a steep increase of the primary current, which is concentrated on one or a few primary coils immediately near the secondary coil. A conductive object represents no problem in this case, because it does not constitute a resonant circuit.